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The presence of hazardous gases in confined spaces has been known for thousands of years. Cloaca Maxima, the first wastewater sewer system in ancient Rome, was constructed from 800 to 735 BC. A few privileged Roman citizens had internal latrines connected directly to the sewer and the odor of “noxious fumes” escaping into the household is well documented. Criminals were often employed to enter the sewer system for cleaning because it was a deadly chore.

Today, it’s possible to detect hazardous atmospheric conditions in confined spaces using an appropriate safety gas detector, yet hundreds of workers still perish in confined spaces every year. The National Institute for Occupational Safety and Health (NIOSH) estimates that millions of workers globally may be exposed to hazards in confined spaces. There are more than 4.8 million estimated permit-required confined spaces in the United States alone.

According to data collected by the U.S. Department of Labor, fatal injuries in confined spaces have fluctuated from a low of 81 in 1998 to a high of 100 in 2000, averaging 92 per year over that five-year period.

What’s a confined space?
Confined spaces may be encountered in virtually any occupation and recognition of the potential hazards combined with proper training is key to staying safe. According to OSHA, confined spaces include, but are not limited to underground vaults, tanks, storage bins, manholes, pits, silos, process vessels, and pipelines.

OSHA’s term “permit-required confined space” (permit space) describes a confined space that has one or more of the following characteristics: contains or has the potential to contain a hazardous atmosphere; contains a material that has the potential to engulf an entrant; has walls that converge inward or floors that slope downward and taper into a smaller area which could trap or asphyxiate an entrant; or contains any other recognized safety or health hazard, such as unguarded machinery, exposed live wires, or heat stress.

In North America employers can face substantial financial and legal penalties in the wake of workplace injuries and fatalities. OSHA standard 1910.146 contains requirements for practices and procedures to protect employees in general industry from the hazards of entry into permit-required confined spaces. This section does not apply to agriculture, to construction, or to shipyard employment (Parts 1928, 1926, and 1915 of this chapter, respectively).

Prepare for entry
According to OSHA, before an employee enters the space, the internal atmosphere must be tested, with a calibrated direct-reading instrument, for oxygen content, for flammable gases and vapors, and for potential toxic air contaminants, in that order.

Oxygen depletion or enrichment, toxic/poisonous and combustible gases are prevalent atmospheric hazards in confined spaces. These conditions may be present prior to entry or they can arise suddenly any time during occupation of the confined space. Often activity in or near a confined space can contribute to changes in atmospheric conditions.

Once you’ve assessed atmospheric risks, use an appropriate gas detector to monitor the environment. Most entries require monitoring of the atmosphere for oxygen concentration, toxic contaminants and combustible gases. According to OSHA’s standard, the atmosphere within the space must be periodically tested as necessary to ensure that the continuous forced air ventilation is preventing the accumulation of a hazardous atmosphere.

Any employee who enters the space, or that employee's authorized representative, shall be provided with an opportunity to observe the periodic testing required.

The most common configuration for a portable gas detector used in routine confined space entry includes oxygen, combustible (%LEL), hydrogen sulfide and carbon monoxide sensors. But not all applications are the same, and the types of sensors selected should include the known and potential atmospheric hazards associated with that particular space. Today’s detectors offer a wide selection of sensor options.

Testing stratified atmospheres
When sampling the confined space atmosphere, keep in mind that all gases have a vapor density. In comparison to normal air, some gases are heavier, some are lighter and some have a comparable density. Based on vapor density, gases in a confined space will stratify.

For example:

Methane is lighter than air and tends to collect in pockets at the top of a space;

Oxygen is a component of air and can be found at all levels;

Carbon monoxide is relatively close to the density of air and can be encountered at all levels;

Hydrogen sulphide is heavier than air and tends to collect at the bottom.

OSHA requires when monitoring for entries involving a descent into atmospheres that may be stratified, the atmospheric envelope should be tested a distance of approximately 4 feet (1.22 m) in the direction of travel and to each side. If a sampling probe is used, the entrant's rate of progress should be slowed to accommodate the sampling speed and detector response.

Evaluation testing
According to OSHA, the atmosphere of a confined space should be analyzed using equipment of sufficient sensitivity and specificity to identify and evaluate any hazardous atmospheres that may exist or arise, so that appropriate permit entry procedures can be developed and acceptable entry conditions stipulated for that space.

Evaluation and interpretation of these data, and development of the entry procedure, should be done by, or reviewed by, a technically qualified professional (such as OSHA consultation service, or certified industrial hygienist, registered safety engineer, certified safety professional, certified marine chemist, etc.) based on evaluation of all serious hazards.

The atmosphere of a permit space which may contain a hazardous atmosphere should be tested for residues of all contaminants identified by evaluation testing using permit specified equipment to determine that residual concentrations at the time of testing and entry are within the range of acceptable entry conditions. Results of testing (such as actual concentration, etc.) should be recorded on the permit in the space provided adjacent to the stipulated acceptable entry condition, according to OSHA.

Measurement of values for each atmospheric parameter should be made for at least the minimum response time of the test instrument specified by the manufacturer, according to OSHA.

OSHA’s Appendix B to the 1910.146 standard states that a test for oxygen is performed first because most combustible gas meters are oxygen dependent and will not provide reliable readings in an oxygen-deficient atmosphere. Combustible gases are tested for next because the threat of fire or explosion is both more immediate and more life threatening, in most cases, than exposure to toxic gases and vapors. If tests for toxic gases and vapors are necessary, they are performed last.

In a safe space
Once workers enter a confined space that has been declared safe, continuous monitoring of the atmosphere is essential, as the OSHA standard mandates. Ideally, each worker in a confined space should be outfitted with a personal multi-gas detector. A variety of standard four-gas detectors are available that are economical, compact and lightweight, making it more affordable for companies to protect their employees and more comfortable for workers to wear the monitors.

The ideal confined space gas detection package should include an appropriate multi-gas detector with an integral motorized pump for initial investigation and attendant monitoring, and individual small, lightweight diffusion multi-gas detectors for each employee.

It is incumbent upon the employer to provide workers with a safe environment to perform their duties and comply with 1910.146. Employees should feel empowered to implement safe work behaviors and to report any work practices they feel are not safe. Armed with the right equipment and the knowledge that comes from proper training, confined space workers can return home safely after a hard day’s work.

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NFPA ® for Electrical Safety in the Workplace is revised every three years, providing the most up-to-date requirements for safe work practices to reduce exposure to electrical hazards. This program analyzes several significant changes in 70E ® and is designed to clarify the reasoning behind the changes, and assist in determining how the changes impact employees and employers.